This proposal concerns optical and mechanical measurements of skeletal muscle derived from an order of magnitude fewer sarcomeres than previously examined. The new information obtained from single fibers and muscle bundles will offer fundamental insights into the dynamics of contractile processes at the cellular, sarcomere and cross-bridge levels. Our optical technique allows detailed investigation of the 1st order diffraction line intensity distributions. Preliminary results show that local sarcomere length can be defined with high spatial and time resolutions (better than 0.1 nm and 1 ms, respectively), and that the following experimental aims are feasible: 1) to correlate the diffraction line peaks with the molecular organization of the contractile filaments; 2) to associate the sub-cellular muscle morphology with the observed diffraction line pattern; 3) to establish the possible relationship between observed sarcomere length 'hesitations' and the myosin head translation during head rotation; 4) to provide direct measures of the rates of attachment, detachment, and re-attachment of the cross-bridges. The local sarcomere length signals will be used to advantage in conjunction with force data to measure the visoelastic moduli (from pseudorandom white noise (PRWN) tests) and stiffnesses (from PRWN and step tests) of different sarcomere population sizes, and the distribution of these measures along the fiber length. The combined optical and mechanical data will afford unique information with which to challenge and test sliding filament theories of contraction. The new data will furnish a fresh framework for the development of an improved model of contraction mechanisms.